So Valve has made Portal free again, claiming it's part of an effort to make video games more useful to education, saying that "using interactive tools [in education] like the Portal series to draw them [the students] in makes physics, math, logic, spatial reasoning, probability, and problem-solving interesting, cool, and fun." Whether or not Portal in the classroom actually makes a difference (it might?) is up for debate. But it's free, for right now, at http://www.learnwithportals.com/ . And this offer only lasts until 9/20, as the title suggests.

This is of course assuming that there's somebody on this forum board that doesn't already have Portal. If not, carry on. :P